Torrejonian-Tiffanian Transition of North American Western Interior
Boyer standing East of the Crazy Mountains, Montana
The period of interest can be referred to as the Early-Late Paleocene transition. The Paleocene (65-55 mya) represents the first ecologic boom time for mammals. Once the dinosaurs went extinct, there was nowhere to go but up for the meek mammals that existed prior to dinosaur extinctions. Thus mammalian ecological diversity appears to have steadily increased through this period. Although much is known about the Early Paleocene and the Late Paleocene, little of the “middle” Paleocene transition is known. Given the dearth of information on the transitional interval itself, hypotheses regarding the nature and mechanism of this change have not yet been thoroughly addressed. The major hypothesis that cooling and aridification during the Late Paleocene lead to biological diversity drops, southern migrations and range constrictions, as well as somewhat more directional evolution leading to “pseudoextinction” of many taxa. However, even our initial results and re-consideration of existing data are beginning to question these perspectives. New discoveries of certain mammalian groups suggest “mixing” more than “shifting” of ranges as there is beginning to be ample evidence of northward migrations as well (St. Clair et al. 2010). To begin evaluating such suspicions in more detail we need to complete sampling at the richest known Early-Late Paleocene transition quarry discovered by us in 2005 (Donald Quarry) and to begin studying these new samples in depth.